Color photographic elements are conventionally formed with superimposed blue, green, and red recording layer units coated on a support. The blue, green, and red recording layer units contain radiation-sensitive silver halide emulsions that form a latent image in response to blue, green, and red light, respectively. Additionally, the blue recording layer unit generally contains a yellow dye-forming coupler, the green recording layer unit generally contains a magenta dye-forming coupler, and the red recording layer unit generally contains a cyan dye-forming coupler.
Following imagewise exposure, a negative working photographic element is processed in a color developer that contains a color developing agent that is oxidized while selectively reducing to silver the latent image bearing silver halide grains. The oxidized color developing agent then reacts with the dye-forming coupler in the vicinity of the developed grains to produce an image dye. Yellow (blue-absorbing), magenta (green-absorbing) and cyan (red-absorbing) image dyes are formed in the blue, green, and red recording layer units, respectively. Subsequently the element is bleached (i.e., developed silver is converted back to silver halide) to eliminate neutral density attributable to developed silver and then fixed (i.e., silver halide is removed) to provide stability during subsequent room light handling.
When processing is conducted as noted above, negative dye images are produced. To produce corresponding positive dye images, and hence, to produce a visual approximation of the hues of the subject photographed, white light is typically passed through the color negative image to expose a second color photographic material having blue, green, and red recording layer units as described above, usually coated on a white reflective support. The second element is commonly referred to as a color print element. Processing of the color print element as described above produces a viewable positive image that approximates that of the subject originally photographed.
A positive working color photographic element is first developed in a black-and-white developer where the exposed crystals are selectively reduced to metallic silver. The unexposed silver is then fogged and reduced by a chromogenic color developer in a subsequent step to generate cyan, magenta, and yellow image dyes. The film is further bleached and fixed as with the negative working film. The positive working film thus forms dyes in the unexposed areas and renders a positive image of the scene, directly.
A problem with the accuracy of color reproduction delayed the commercial introduction of color negative elements. In color negative imaging, two dye image-forming coupler containing elements, a camera speed image capture and storage element and an image display, i.e., print element, are sequentially exposed and processed to arrive at a viewable positive image. Since the color negative element cascades its color errors forward to the color print element, the cumulative error in the final print is unacceptably large, absent some form of color correction. Even in color reversal materials which employ just one set of image dyes, color correction for the unwanted absorption of the imperfect image dyes is required to produce acceptable image color fidelity for direct viewing.
The complicated processing can be eliminated by substituting direct positive emulsions for the negative-working silver halide emulsions conventionally present in color reversal films. Unfortunately, direct positive emulsions are more difficult to manufacture, exhibit lower levels of sensitivity at comparable granularity, and have unique problems of their own, such as re-reversal, that have almost entirely foreclosed their use as replacements for negative-working emulsions.
Radiation-sensitive silver halide grains possess native sensitivity to the near ultraviolet region of the spectrum, and high bromide silver halide grains possess significant levels of blue sensitivity. Blue recording layer units often rely on the native sensitivity of the high bromide silver halide emulsions they contain for light capture. Blue recording layer units sometimes and green and red recording layer units always employ spectral sensitizing dyes adsorbed to silver halide grain surfaces to absorb light and to transfer exposure energy to the radiation-sensitive silver halide grains. In a simple textbook model the light absorbed in each of the blue, green and red recording layer units is limited to just that one region of the spectrum. For blue, green and red recording layer units light absorption in the blue (400 to 500 nm), green (500 to 600 nm) and red (600 to 700 nm) spectral region, respectively, is sought.
In practice each spectral sensitizing dye exhibits a peak (occasionally a dual peak) absorption wavelength and absorption declines progressively as exposure wavelengths diverge from the peak. Thus, considerable effort has gone into selecting spectral sensitizing dyes and dye combinations that best serve practical imaging needs.
The use of spectrally sensitized tabular grain emulsions in the minus blue recording layer units of color photographic elements has been demonstrated by Kofron et al U.S. Pat. No. 4,439,520 to improve image sharpness and to increase speed in relation to granularity. Kofron et al demonstrates that improvements in performance are realized as the average aspect ratios of the tabular grain emulsions are increased.
Kofron et al further discloses a variety of layer arrangements for color photographic elements having blue, green and red recording layer units, including arrangements containing two or more of each of green and red recording layer units differing in speed. Other illustrations of color photographic elements containing two or more green and/or red recording layer units are provided by Research Disclosure, Vol. 389, September 1996, Item 38957, XI. Layers and layer arrangements.
The green sensitivity of a multilayer film element is determined by the light absorption profile of the silver halide emulsions in the green sensitive layer unit attenuated by any light absorbing materials that lie above it in the top layers of the film, such as ultraviolet filter dyes, Lippmann emulsions, yellow filter layers, the blue sensitive emulsions, the yellow and magenta colored masking couplers in color negative films, and the optical properties of the red sensitive emulsions underneath the green record. The light absorption of the emulsions used in the green sensitive layer unit is in turn determined by the composite absorption of the specific combination of spectral sensitizing dyes adsorbed to the surface of the silver halide crystals, since silver halide emulsions only have native (intrinsic) sensitivity to blue light. Green sensitive emulsions used in the green recording layer unit that are commonly found in the art are observed to employ two or three green sensitizing dyes, and typically peak in dyed absorptance from about 530 nm to about 560 nm.
It has long been recognized that different light sources may require accommodation by the photographic system. For example, tungsten lighting sources emit substantially more light in the red-sensitive color band than in the blue-sensitive color band. Professional photographers have adapted to tungsten light in one of two ways. A daylight-balanced film, which is designed for a uniform (daylight) light source may be successfully used with tungsten lighting by fitting the camera with filters that remove some of the tungsten-lit scene's red and green light. This approach effectively “slows” the film speed and balances the light received by each layer, making the light look “uniformly spectrally distributed” to the film. Alternatively, a “tungsten” film, specifically designed to be used with tungsten light sources, may incorporate a faster blue layer and a slower red layer. This approach unbalances the speed of the film's layers to counteract the unbalance in the light source.
Amateurs photographing with tungsten illumination encounter varying results. With a daylight balanced reversal film, a tungsten-illuminated image takes on orangy-red hues. With a daylight balanced color negative film the tungsten-illuminated image yields an unbalance in the colors on the negative which may be accommodated by using filters during print-making to give a neutral print.
Fluorescent lights are quite different from tungsten in both design and quality of light. Fluorescent lights operate when an electric current passes through a tube filled with mercury gas. The excited mercury atoms emit visible and UV light. A strong visible emission from the mercury gas occurs at 545 nm, in the center of the color green. This emission gives fluorescent lights a greenish tinge, common to all fluorescent lights.
Designers of fluorescent lighting try to minimize the perception of the green emission by including phosphors, which absorb the UV and blue portion of the mercury emission and emit other colors. Among the “white” tubes commonly used for lighting many variations exist. “Cool white,” for example, has more red than “Daylight” tubes. The variation in color is determined by the phosphors. But the quality of the visible emission of fluorescent lights is dominated by the green mercury emission at 545 nm.
It is possible that a film could be designed for fluorescent lighting by modifying and unbalancing the speeds of the film's layers to counteract the unbalance in the fluorescent light source. This approach could create a film specifically for fluorescent lighting, with altered speeds in the various color records. A film designed in this manner would be unbalanced for use in the uniform lighting unless coupled with a colored filter to make up for the unbalance.
Previous workers have tried to design green spectral sensitivities for color negative films for the purpose of reducing illuminant sensitivity. U.S. Pat. No. 3,672,898 describes a film with sensitivities designed to work with sunlight, tungsten, and fluorescent lighting. This film uses specific spectral sensitization in combination with ultraviolet, yellow, and magenta filter dyes. This film is complex to manufacture and does not yield saturated colors because of poor interimage correction.
U.S. Pat. No. 5,166,042 describes a color photographic film that is designed to have improved color reproduction under fluorescent lighting. The film features a spectral sensitivity such that the sensitivity measured with a monochromatic light source at 560 nm is such that the speed difference between the green and red recording units is in the range from −0.2 to 1.0. Most films have a larger speed separation at this wavelength. The difficulties with this approach are that much more green-red interimage correction would be required for a film with this characteristic, and this approach still does not address the problem of too much green speed and not enough blue speed.
U.S. Pat. No. 5,200,308 also describes color film spectral sensitivities designed to improve color reproduction under fluorescent illumination. The specified sensitivities increase the red and blue response of the film to fluorescent light sources, but the high green sensitivity of the film at the emission line of the fluorescent light source, limits the amount of improvement that can be achieved. U.S. Pat. No. 5,258,273 specifies a red spectral sensitivity of a multilayer color film structure. Color reproduction can be improved by increasing the short wavelength red response to better match the red phosphors used in fluorescent lights, but again the amount of improvement is limited because the green sensitivity is still higher than the red and blue under fluorescent illumination.
European Patent Applications 447 138 A1 and 458 315 A1 both describe green spectral sensitivities useful for photographs taken under fluorescent illumination. The sensitivity at 545 nm is reduced by shifting the peak sensitivity to a shorter or longer wavelength. However, the sensitivity has a single peak, and therefore, the aggregate green spectral sensitivity is not centered in the green region of the spectrum. Even though the green response of the film to fluorescent lighting is reduced, the shift of the overall green spectral sensitivity will have an undesirable effect on hue reproduction and the reproduction of skin colors.
U.S. Pat. Nos. 6,093,526 and 6,296,994 describe a preferred emulsion green absorptance and a preferred color film spectral sensitivity, respectively. These sensitivities are modeled after human eye sensitivities and should capture images with less sensitivity to illuminant changes. However, these sensitivities are intended for a film that is to be scanned and printed digitally. The high degree of overlap between the color records makes it impossible to achieve saturated colors with these spectral sensitivities when the film is printed by conventional optical printing methods.
U.S. Pat. No. 6,479,226 describes a green-sensitive element which gives a double peak, one in the 525 to 540 nm region and one in the 550 to 565 nm region. This method provides a broadened green-sensitive spectral envelope and is much like that of U.S. Pat. No. 5,053,324, and U.S. Pat. No. 5,308,748. Though these create maximum absorptions removed from the spiked green illuminant, none of these spectral envelopes sufficiently reduces absorption in the region of the 545 nm spike.
U.S. Pat. Nos. 4,705,744; 4,707,436, and 5,035,324 use a fourth, non-imaging layer with a spectral sensitivity between blue and green. This layer releases chemical inhibitors to adjust the response of imaging layers. The degree to which this happens depends on light distribution. This approach, as currently practiced, cannot adjust the film's response adequately over the whole tone scale, and probably suffers image structure degradation as a result of the presence of the fourth layer.